chart-value

Returns an individual value that is in a chart-widget. This could be a value that was
added by calling set-chart-value or a value that's returned by
the widget's chart-value-returner function if it has one.
The value will be nil if the chart does not
contain a value for the specified item, object, and value-type, and
otherwise it will be a number.

chart-widget is the widget to be accessed.

The keyword arguments are:

item-index is the zero-based index of the
chart item to access, or nil to not specify
an item index. Either this argument or item-id
must be true. item-index takes precedence if both
are true. The widget's
chart-item-id-equality-test is
used to find a matching item-index.

object-index is the zero-based index of
the chart object to access, or nil to not
specify an object index. The default is zero if neither this argument
nor object-id is true.
object-index takes precedence if both are true.
The widget's
chart-object-id-equality-test
is used to find a matching object-index.

item-id is the arbitrary ID of the chart
item to access, or nil to not specify an ID.

object-id is the arbitrary ID of the chart
object to access, or nil to not specify an ID.

value-type is the type of to return. This
argument may be one of the following keywords:

:value: the "main" value of a chart item and object
pair.

:from-value: an optional starting value for a bar in
a bar graph.

:high-value: an optional value that's normally
greater than the main value.

:low-value: an optional value that's normally less
than the main value.

:icon-size: an optional size for a built-in icon
drawn on a line graph.